This application concerns a radically new and improved color television picture tube of the shadow mask variety and methods for making same. As used herein, the term "shadow mask" is intended to encompass all tubes, including post deflection focus tubes, in which a color selection mask or electrode achieves a shadowing effect, whether total or only partial.
The shadow mask color television picture tube, now mass produced world-wide, emerged in the 1950's as the favorite from a group of proposed color tube types. Significant improvements occurred in rapid succession. Tube brightness, at first inadequate for all but darkened room viewing, is now sufficient for most conceivable ambient lighting conditions, due in large part to the introduction by the assignee hereof of negative guardband, black surround tubes which provided greater contrast and twice the brightness of earlier tubes. The negative guardband, black surround principle is disclosed and claimed in U.S. Pat. No. 3,146,368 issued to Joseph P. Fiore and Sam H. Kaplan and owned by the assignee hereof. Recent trends include increased rectangularity of the viewing area and a gradual change from the 90.degree. deflection angle tube of the 1960's to the slimmer, wider angle tubes in the 1970's. In spite of the marked improvements in color tube performance over the years, significant reductions in the cost of manufacturing color tubes were achieved.
This invention is directed to the provision of a revolutionary next generation color picture tube having a novel construction and improved manufacturing methods which makes possible significant further improvements in tube performance and even lower cost of manufacture.
The manufacture of shadow mask color tubes, at least that part with which this invention is most directly concerned, involves the making of the shadow mask, the forming of the phosphor screen on the faceplate portion of a glass envelope, and the assembly of mask and screen. In the manufacture of conventional shadow mask color tubes, a flat shadow mask blank is coated on both sides with a layer of photoresist material; registered mask master stencil patterns are then contact printed on the opposed photoresist layers. After development of the photoresist layers, the blank is etched from both sides to form a pattern of mask apertures in a central region of the blank. The apertured mask blank is then "formed" by a metal stamping or drawing process to a three-dimensionally curved shape, typically a compound spherical (multi-radial) shape.
The formed shadow mask is then welded on a heavy, rigid frame. The mask assembly is ultimately suspended in a tube with the mask spaced about 1/2 inches from the phosphor screen of the tube; the screen takes the form of a mosaic pattern of red-emissive, blue-emissive and green-emissive phosphor element triads. The mask serves to "shadow" the phosphor screen such that each of three electron beams carrying red, blue and green color information each "see" only red, blue and green phosphor elements, respectively.
In a typical large screen color television tube, there are approximately 400,000 apertures which must be aligned exactly with a corresponding pattern of 400,000 phosphor element triads. In the manufacture of conventional color tubes, in order to assure that each mask aperture is precisely aligned with its associated triad of phosphor elements, in spite of irregularities in the mask aperture patten (which may be introduced in the mask forming or etching processes, during handling, etc.), the shadow mask is used as a photographic stencil during the photoexposure operations employed to form the phosphor screen. Thus, in each tube, a unique shadow mask aperture pattern is replicated into the pattern of phosphor element triads which collectively constitute the phosphor screen. The same mask which was used in the photoprinting of a particular phosphor screen must, of course, be ultimately mated or "paired" with the screen. This demands that each mask follow the faceplate carrying its mating screen throughout the tube factory -- a logistical bete noire.
The color television tube which has become standard, particularly in large screen sizes, has a so-called "negative guardband", "black surround" screen. In this type of screen the electron beam lands are caused, by appropriate sizing of the mask apertures and phosphor elements, to be larger than the impinged phosphor elements by an amount equal to the allotted beam landing tolerance or "guardband". This type of screen is further characterized by having black material between the phosphor elements for enhanced contrast. It is standard practice in the manufacture of such tubes to first deposit on the inner surface of the tube faceplate a black "grille", i.e., a layer of light-absorptive material having openings in which the phosphor elements are to be subsequently deposited. The black grille and three patterns of phosphor elements (red-emissive, blue-emissive and green-emissive) are deposited in succession on the faceplate by photochemical methods which involve coating a light-sensitive layer on the faceplate and exposing the layer through a uniquely associated shadow mask to a source of light actinic to the layer. Such an exposure operation is herein termed a "photoexposure" operation.
In the exposure of the photosensitive coating used to make the black grille, the coating is exposed to point or line light sources (depending on the tube type) at three locations simulating the ultimate electron beam deflection center locations. In the screening of the red-emissive, blue-emissive and green-emissive phosphor elements, a single exposure is made from a different one of the three light source locations. The proper selection of the mask-to-faceplate spacing and the location of the light sources are selected to assure the proper parallax relationship of the electron beam sources, the mask and the phosphor screen when the end-product tube is finally assembled. The necessarily large spacing of the shadow mask from the screen, however, makes it difficult to accurately form the grille openings, and thus to accurately form the phosphor elements which fill the openings, and inevitably results in undesirably long exposure times.
In tubes of the negative guardband, black surround type, as explained, the electron beam landing spots are larger than the impinged phosphor elements. Since in conventional practice the shadow mask is used as the exposure stencil during the photoexposure operations used to screen the faceplate, some method must be provided for causing the electron beam spots to be larger than the impinged phosphor elements. Two methods are employed commercially. The first is the so-called "re-etch" or "etch-back" method wherein the shadow mask apertures are originally formed to the (smaller) size of the phosphor elements, and then after the screening operations, the shadow mask is "re-etched" (etched a second time) until the shadow mask apertures are larger than the phosphor elements by the allotted tolerance value, thus producing the desired negative guardband condition.
The second method used to cause the mask apertures to be larger than the associated phosphor elements is to use a shadow mask which has full-sized apertures and, by the use of special photoreduction techniques during the photoexposure operations, to cause the phosphor elements to be smaller than the shadow mask apertures. The former method suffers from its requirement of an additional mask etching operation. The latter method is difficult to execute due to the very tight tolerances necessarily imposed during the photoreduction operations to assure the proper sizing of the phosphor elements.